The present invention relates to a steering apparatus for a vehicle and a method for controlling the same.
A steering control system, which controls the yaw moment of a vehicle by controlling the steering angle of steered wheels based on a vehicle model (vehicle motion model), has been proposed in recent years (refer to, for example, Japanese Laid-Open Patent Publication No. 2002-254964). The vehicle model is generated by modeling the relationship between vehicle state parameters, such as vehicle velocity and yaw rate, and motion states of the vehicle. An intelligent front steering (IFS) system is one example of such a steering control system.
The IFS system integrally executes electronic control associated with steered wheels, such as variable gear ratio control and power assist control, based on a plurality of vehicle state parameters. The variable gear ratio control changes the transmission ratio (gear ratio) of the steered wheels with respect to the steering angle (steering angle) of the steering wheel. The power assist control applies an assist force to the steering system of the vehicle.
For example, the IFS system calculates target values (e.g., a target yaw rate) of vehicle-condition parameters based on the vehicle model. Based on the target values, the system determines the steering characteristic of the vehicle and calculates a control amount for the steered wheels. When the vehicle is in an understeer state, the system executes such control that reduces the turning angle of each steered wheel (understeer control). When the vehicle is in an oversteer state, the system executes such control that steers each steered wheel at an angle in a direction opposite the direction of the yaw moment, that is, executes control that for counter-steering (oversteer control). This stabilizes the trajectory of the vehicle even under conditions where the trajectory of the vehicle is likely to become unstable, such as on a low-friction road.
However, errors in the vehicle model calculation may often occur when the vehicle velocity is very low (e.g., 5 km/h and lower). Thus, excessive control tends to occur in such a low vehicle velocity range. Such excessive control may cause the vehicle to sway even though it is traveling straight in a normal state. Accordingly, in the low vehicle velocity range, the control of the steering angle of the steered wheels based on the vehicle model, such as the oversteer control and understeer control, are not executed in the prior art.
Under some conditions, however, the trajectory of the vehicle may become unstable when the vehicle velocity is extremely low. For example, when oversteer control is being executed to stabilize the trajectory of the vehicle, the control may be ended due to decrease in the velocity of the vehicle. Improvements should be made with respect to this point.